Electronic devices are constantly being made smaller to decrease their production costs and increase their operating speeds. To develop suitable new manufacturing processes and to correct defects in existing processes, engineers and scientists need analytical instruments that can create images of extremely small features and determine the chemical make-up of those features. Scanning electrons microscopes (SEMs) are widely used for imaging microscopic features, and Auger Electron Spectroscopy (AES) is often used to determine the chemical make-up of feature surfaces. AES can detect the presence of the lighter elements in quantities as small as a few atomic layers or less. In Scanning Auger Microscopy (SAM), analyses can be made in three modes: spectral analysis, single element maps, and depth profiles using simultaneous ion milling and Auger analysis.
In an SEM, a finely focused primary electron beam is scanned over a specimen. The impact of the primary electron beam causes the ejection of electrons, referred to as secondary electrons. An image of the target is then formed in which the brightness of each point on the image is determined by the number of secondary electrons ejected while the primary electron beam was impinging at that point. The finely focused electron beam of an SEM allows for the production of an image of greater magnification, greater depth of focus, and higher resolution than can be achieved by the best optical microscopes.
Secondary electrons are ejected from the specimen by a variety of mechanisms. One of those mechanisms, the Auger process, produces electrons having energies characteristic of the material from which the electronics are ejected. Electrons produced by the Auger process are known as Auger electrons. AES is the process of analyzing the energy of the Auger electrons and determining the type of material from which the electrons were emitted.
Unfortunately, only a small number of the impacting electrons give rise to Auger electrons. Typically, somewhere between one thousand and one hundred thousand primary electrons are required to produce one Auger electron. To detect a material present in the sample at very low concentrations, it is necessary therefore to efficiently collect and analyze the Auger electrons. Auger electrons are emitted nearly isotropically, that is, approximately equally in all directions above the target, so it is necessary to collect Auger electrons from as much of the hemisphere above the sample as possible.
FIG. 1 shows schematically a conventional SEM 1 with an AES system 2. An electron column 3 directs a primary electron beam 4 toward a sample 5. Secondary electrons 6 are collected through an opening 7 to the side of the primary beam and transferred to an analyzer 8. Because the objective lens of electron column 3 takes up much of the space near the sample, it is difficult to position opening 7 sufficiently near the sample to collect more than a small portion of the Auger electrons. Since opening 7 collects electrons from only a small portion of the hemisphere above the sample, it is not an efficient collector, and side-mounted AES systems on SEMs are not very sensitive, that is, they are not able to detect extremely small concentrations of a material. Moreover, such side-mounted electron collectors are incompatible with high resolution electron lenses for SEMs, because the magnetic field from the lenses greatly reduces the number of electrons collected.
Many attempts have been made to increase the sensitivity of AES systems by increasing the percentage of collected Auger electrons. For example, U.S. Pat. No. 4,810,880 issued to one of the present applicants discloses a Direct Imaging Auger System that uses a wide electron beam from a side-mounted source to illuminate a large area on a sample. The electrons emitted from the sample are then collected using a high resolution “snorkel” electron lens pole piece positioned immediately below the specimen. A secondary electron optical system forms an image of the sample using the secondary electrons, and the electrons from different parts of the sample can be separated and analyzed to determine the types of materials present at different locations on the sample. This type of instrument, in which the primary beam impacts and forms an image of a large area of the sample, is referred to as an “imaging” instrument, as opposed to a “scanning” instrument, which illuminates only a small point of the sample at one time, collects electrons from that one point, and then combines the information for all scanned points to create an image. In an imaging instrument, the resolution depends primarily how faithfully the collection system can reproduce an image of the specimen, whereas in a scanning system, the resolution depends primarily on the size of the illuminating beam. An imaging instrument typically cannot collect Auger electrons from each point as efficiently as a scanning instrument and so the primary beam current density is increased to produce additional Auger electrons. The higher beam current can cause more sample damage than a scanning instrument would cause at comparable beam energies.
In another approach, a Low Energy Electron Microscope (LEEM) System is described in “Spectroscopy in a Low Energy Electron Microscope” by E. Bauer, C. Koziol, G. Lilienkamp, and T. Schmidt, Journal of Electron Spectroscopy and Related Phenomena, Vol. 84, pp. 201-209 (1997). The Bauer et al. instrument provides Auger spectra and images of surfaces. This is also an imaging type instrument as opposed to a scanning instrument and is very complex. The demonstrated Auger image resolution for silver is about 100 nm, which is relatively low for use in semiconductor industry applications.
A Transmission Electron Microscope (TEM) is an imaging instrument that uses a high energy electron beam and forms an image of the sample using electrons that are transmitted through the sample and collected on the opposite side. A Scanning Transmission Electron Microscope (STEM) also uses electrons transmitted through the sample, but scans a high energy beam across the sample, rather than illuminating the entire sample area simultaneously. Such instruments have high resolution but, because electrons must go completely through the sample, can be used only with very thin samples. It is known to collect secondary electrons back through the lens of the primary electron column of a TEM. Such a system is described by Kruit in “Auger Electron Spectroscopy in the STEM,” Quantitative Microbeam Analysis, Proc. of the 40th Scottish Universities Summer School in Physics, August,1993 ISBN 0-7503-025 6-9, p. 121-143. Such systems employ an objective lens that produces a strong magnetic field which “parallelizes” the secondary electrons, that is, the magnetic field changes the trajectories of the secondary electrons from a widely dispersive pattern to an almost parallel pattern, as they are transmitted up through the lens pole piece. Beyond the lens, a magnetic deflector or a combination magnetic and electrostatic deflector, such as a Wien filter, deflects the secondary electrons to the side of the primary beam towards an Auger electron energy analyzer.
The secondary Auger electrons can be readily separated from the primary beam electrons because of the large difference in the energy between the Auger electrons and the primary beam electrons. Electrons in the primary beam of a TEM have an energy of about 200 keV and the Auger electrons have energies about 50 eV to 3000 eV energy. With this large difference between primary beam electron energy and Auger electron energies, a magnetic field can separate the Auger electrons with minimal aberration of the primary electron beam.
The desirability of implementing a similar through-the-lens Auger electron system in an SEM has been recognized, and attempts to create such a system are described, for example, by P. Kruit in “Magnetic Through-the-Lens Detection in Electron Microscopy and Spectroscopy, Part 1,” in Advances in Optical and Electron Microscopy, Vol. 12 ed., Mulvey and Sheppard, Academic Press, pp.93-137 (1991). Such attempts have met with limited success.
A primary problem in incorporating through-the-lens collection of secondary electrons in an SEM is the difficulty of separating the secondary electrons returning through the lens along the same path as the primary beam electrons. The electrons in the primary beam of an SEM typically have energies of between 3 keV to 30 keV, which is significantly less than the typical 200 keV energy of electrons in a TEM primary beam and is much closer to the 50 eV to 3,000 eV energy of the Auger electrons. The separating or deflecting device used in the TEM Auger systems to separate the electrons transmitted back through the lens, while producing minimal aberrations in the high energy TEM beam, can cause severe aberrations in the lower energy SEM primary beam, thereby reducing the resolution and usefulness of the SEM. Although the aberrations can be reduced by known methods, such as designing a primary beam crossover in the magnetic deflector or Wien filter device, such solutions restrict the optical flexibility of the primary beam optics, since such solutions tend to increase primary beam aberrations from another source, that is, beam interactions. Moreover, such magnetic deflector devices have hysteresis, which can adversely affect performance.
Consequently, there is a need for a method and apparatus that combines SEM imaging and Auger electron spectroscopy and that provides maximum performance of both the SEM and AES functions.